Engines may be operated with variable valve timing control to improve engine performance. For example, intake and/or exhaust valve timings may be adjusted (e.g., advanced or retarded) based on engine operating conditions to increase positive valve overlap. The increased valve overlap may then be used for improving air-fuel mixing, cylinder charge temperature control, etc. During still other conditions, the valve timings may be adjusted to increase negative valve overlap.
One example approach for controlling an engine variable valve timing device is shown by Winstead in U.S. Pat. No. 7,779,823. Therein, a valve timing of a first engine cylinder is adjusted to flow gases from the engine intake to the engine exhaust while a valve timing of a second cylinder on the same bank is adjusted to return combusted exhaust gases back to the intake. In this way, a timing of an auto-ignition combustion in the engine (e.g., when operating in an HCCI mode) can be controlled.
However, the inventors herein have identified potential issues with such an approach. As one example, cooled EGR may not be achieved. Specifically, since exhaust gases are returned via one or more cylinders of the same bank, the returned exhaust gas may be at a substantially high temperature. While this may help expedite cylinder heating while the engine is in a HCCI combustion mode, during a spark ignition combustion mode, the higher temperature recycled exhaust gases can lead to misfires and other abnormal combustion events in the combusting cylinders. As such, this may degrade engine performance.
Thus in one example, some of the above issues may be addressed by a method of operating an engine comprising operating a first group of cylinders on a first engine bank to provide a net flow of air and exhaust gas from a first intake manifold to a first exhaust manifold while operating a second group of cylinders on a second engine bank to provide a net flow of exhaust gas from a second exhaust manifold to a second intake manifold. In this way, fuel may be combusted on a first activated bank of cylinders while exhaust gas is recirculated via a second deactivated bank of cylinders.
For example, an engine may include a first group of cylinders coupled to a first exhaust catalyst on a first engine bank and a second group of cylinders coupled to a second exhaust catalyst on a second engine bank. During selected conditions, such as when an engine load is lower than a threshold, fuel may be injected to, and combusted in, the first group of cylinders. In addition, a valve timing of the first group of cylinders may be adjusted so as to flow air and exhaust gas from an intake manifold towards an exhaust junction through the first exhaust catalyst. At the same time, no fuel may be injected to the second group of cylinders. Instead, a valve timing of the second group of cylinders may be adjusted so as to recirculate at least some exhaust gas from the exhaust junction to the intake manifold via the second group of cylinders. That is, flow through the second bank may be in a direction opposite from the flow through the first bank. As such, the exhaust gas may be cooled as it passages through the cylinders of the deactivated bank, thereby providing cooled EGR. Optionally, an exhaust air-to-fuel ratio at a position between the second exhaust catalyst and the exhaust junction may be monitored to identify the presence of exhaust leaks.
Additionally, temporary enrichment of the exhaust generated at the first group of cylinders may be advantageously used to at least partially regenerate the second catalyst coupled to the second group of cylinders. This reduces the fuel required to regenerate the catalyst upon subsequent cylinder reactivation.
In this way, combusted exhaust gas generated on a first engine bank may be recirculated via a second, different engine bank. By reversing flow through cylinders of a deactivated bank, the recirculated exhaust gas may be cooled. Overall, cylinder deactivation and cooled EGR benefits can be simultaneously provided to improve engine performance. By using the reverse flow to detect exhaust leaks, exhaust degradation may also be diagnosed concomitantly.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.